Effect of soil ameliorant (humic acid) on ethanol yield components of sweet sorghum (sorghum bicolor [l.] moench) cultivars

Abstract

The sweet sorghum plant is one of the heavy feeders on the soil and its roots can penetrate the soil up to 1 to 2 m deep. This often forces farmers to apply huge amounts of synthetic (inorganic) fertilisers to try and compensate for the nutrient requirements of the crop, whilst achieving high yields. Therefore, increasing performance per hectare is now the solution to increasing agricultural produce or production which is achieved through the proper combination of water, soil, soil nutrition, air and temperature. The main aim of the study was to understand the effects of HA application under dryland field conditions on agronomic performance, juice quality and yield traits of three sweet sorghum cultivars grown for ethanol production in the Free State and Potchefstroom. The specific objectives were (i) to investigate how different levels of HA affects growth, development, juice quality and yield of sweet sorghum genotypes grown for ethanol extraction (ii) to investigate any changes in the physical properties of soil and available organic matter when HA is added to the soil, and (iii) to determine the influence of HA on mineral content, antioxidant activity, phenolic content and total flavonoids in three sweet sorghum cultivars. Two field trials were conducted under dryland conditions in the 2022 and 2023 summer growing seasons at the Glen Agricultural Institute and Agriculture Research Council-Grain Crops (ARC-GC) Experimental farm at Potchefstroom, South Africa. The trial used a factorial design and the layout arrangement was organised in a randomised block design replicated three times. The factors used included locations (Glen and Potchefstroom), three sweet sorghum cultivars (ARC-SS 27, ARC-SS 76, and Hunnigreen), as well as treatment levels of HA (0kg, 5kg, 10kg and 15 kg/ha-1). Soil samples were collected before planting and fertiliser applications were done according to the recommendations from a soil test report. Analysis of variance for the sorghum lines used in the experiment indicated the presence of significant differences in morphological characteristics during the first year. During the first year of planting, there were significant differences at p<0.005 on plant height, stalk weight, fresh bagasse, dry bagasse, dry leaf weight, juice weight, juice volume, brix at flowering, brix at harvesting, fresh panicle, dry panicle, and at days to 50% flowering at Potchefstroom. The juice quality and yield parameters of the sweet sorghum lines, humic acid application, and location were significantly different at p<0.05 during the first year at Potchefstroom. Brix at flowering differed significantly (p<0.05) at Potchefstroom in year two with 18.04% being harvested on Hunnigreen cultivar at 15 kg ha-1 of HA while the least was also on Hunnigreen at 0 kg ha-1, HA having been 10.93%. Plant height (PH) was significantly different (p<0.05) at Glen in year one, with the tallest plant of 3.520 m being harvested on the Hunnigreen cultivar at 15 kg ha-1 of HA while the least was on ARC SS27 at 0 kg ha-1 of HA being 2.587 m. There were no differences (p<0.05) on juice weight at Glen in year one with 0.3500 g being harvested on the ARC-SS 27 cultivar at 15 kg ha-1 of HA, while the least of 0.2000 g was also on Hunnigreen at 0 kg ha-1 of HA. At Glen in year two, with the tallest plant being 2.187 m harvested on the ARC-SS 27 cultivar at 15 kg ha-1 of HA, while the least was on ARC-SS 76 at 0 kg ha-1 of HA being 1.390 m but there were no significant differences. There were significant differences (p<0.05) in juice volume in year two at Glen with the highest yield of 17.97 ml at 15 kg ha-1 of HA on the ARC SS76 cultivar, while the least was 10.67 ml on ARC-SS 27 at 0 kg ha-1 of HA. There were no significant differences on available sucrose, fructose, galactose, trehalose, glucose, maltose after HA application, except only on the total sugars. The significant differences in results of the sweet sorghum varieties were due to differences in their agro-morphology as well as competition for minerals, water, and solar radiation among plants. There were various interactions within the study and these affected various agronomic and juice parameters that were recorded. The season affected plant height, stalk weight, fresh bagasse weight, dry leaf weight, juice volume, brix at flowering, fresh panicle weight, dry panicle weight and the number of days to 50% flowering. The location affected PH, fresh bagasse weight, dry leaf weight, juice volume, brix at flowering, fresh panicle weight, dry panicle weight and days to 50% flowering. Location, cultivar, and HA (LXCXHA) and site, location, cultivar and HA (SXLXCXHA) only affected days to 50% flowering. All three varieties examined (Hunnigreen, ARC SS27, and ARC SS76) differed in agronomic attributes, brix and panicle weight. Hunnigreen proved to be a superior variety, yielding the highest total sugar content. ARC-SS 27 and ARC-SS 76 are important in grain production. This study demonstrates the importance of HA on sweet sorghum and how the various varieties respond with increased levels. Further studies can look at other sweet sorghum varieties and HA in different areas. Results from such research would be necessary useful unravel information and guidelines on the use of HA across diverse agro-ecological zones.